In the manufacture and testing of medical devices, mechanisms are used to radially compress cylindrical devices such as stents, balloons, and catheters. For example, installation of a stent into a catheter balloon is typically done by compressing the stent radially inward onto the balloon with enough pressure to permanently deform the stent to a smaller diameter and to slightly embed the metal stent into the plastic balloon. In another example, a polymer catheter balloon is compressed radially after pleating to wrap it tightly around the catheter shaft. In another example, a self-expanding stent is radially compressed to insert it into a sheath or delivery system. In an example of medical device testing, a stent is radially compressed while the required force is measured, in order to measure the stent's functional relationship between diameter and radial force.
A first type of prior art device includes a radial compression mechanism wherein several similar wedge-shaped dies with planar surfaces are arranged to form an approximately cylindrical central cavity, the wedges being hinged and driven in unison to change the diameter of the cavity. A mechanism of this type is illustrated in FIGS. 1 through 5. Examples of this mechanism are the Crimpfox tool sold by Phoenix Contact GmbH 7 Co. KG (CRIMPFOX UD 6-6, Part Number 1206366), and the “segmental compression mechanism” marketed by Machine Solutions Incorporated, and described in U.S. Pat. No. 6,968,607. In this type of mechanism, the working surfaces of the dies have a wedge shape with two planar surfaces meeting at the tip. The dies do not slide against each other but are rotated around a pivot point. A shortcoming of this type of mechanism is that there exists a gap between adjacent wedges, the size of which varies with the diameter of the cavity in an undesirable way. Typically, the mechanism is specifically designed to provide a desired range of cavity diameters. At the lowest and highest diameters, the dies are tightly wedged against each other (zero gap). As the diameter is increased from the lowest, the gap increases until it reaches a maximum, then decreases until it becomes zero again at the highest diameter, as illustrated graphically in FIG. 5. The diameter range and gap (as a function of diameter) depend on the specific design of the mechanism, particularly the location of the hinge or pivot point of the dies and the diameter of the circle formed by all of the die hinge points in the mechanism. A larger diameter of the hinge point circle results in a smaller maximum gap for a given diameter range. The strict design tradeoffs for this type of mechanism results in a mechanism that must be large to provide a small maximum gap for a given diameter range, or a mechanism that must have a large gap to provide the same diameter range in a small size. Large gaps between the wedges are a disadvantage because they allow space for parts of the compressed device to go into. For example, the metal struts of a stent can move into the gap and be damaged.
A second type of prior art device includes a radial compression mechanism wherein several similar wedge-shaped dies with planar surfaces are arranged to form an approximately cylindrical central cavity, the wedges being attached to linear guides and driven in unison to change the diameter of the central cavity. A mechanism of this type is illustrated in FIGS. 6 through 10. Examples of this mechanism include the mechanism taught by Kokish in U.S. Pat. No. 6,651,478, or the mechanism marketed by Interface Associates Inc. (Model W8FH). In this type of mechanism, the working surfaces of the dies have a wedge shape with two planar surfaces meeting at the tip. The linear motion of the wedges in this mechanism provides a wedge-to-wedge gap that is constant, independent of the cavity diameter, and may be designed to be any desired size (see FIG. 10). A shortcoming of this mechanism is that it typically does not provide a sufficiently accurate positional relationship of the wedge-shaped working ends of the dies. Accurate positional relationship of the dies is important so that the central cavity remains approximately round and provides even compression around the circumference of the compressed device, and so that the largest die-to-die gaps aren't much larger than the average. Because each die is carried on its own linear guide, and all of the guides are attached to a plate or base, many parts and attachments may influence the accuracy (roundness) of the central cavity. Medical device manufacturing and testing often requires an accurately round cavity at diameters as small as 0.5 mm. which is typically not achieved by this type of mechanism.
A third type of prior art includes a radial compression mechanism comprising several similarly-shaped dies arranged to form an approximately cylindrical central cavity, the dies being hinged (or pivoted) and driven in unison to change the diameter of the cavity. The working die surfaces are not planar, but have a specifically-designed shape that makes the gap between adjacent dies an arbitrary function of diameter that may be chosen by the designer. Typically, the gap is chosen to be approximately constant, independent of diameter, and as small as manufacturing tolerances will allow (see FIG. 15). Usually, the hinge point of each die is located approximately on the opposite side of the mechanism from the working tip of the die resulting in concave working surfaces. Examples of this mechanism include the mechanism marketed by Blockwise Engineering (Model RJ). A shortcoming of this device is that geometry of each of the dies in the preferred embodiment is difficult to manufacture accurately in non-metallic materials. Non-metallic dies are often required to limit the scoring or abrasion on the compressed article.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved radial compression mechanism for compressing devices such as stents, catheters, balloons, and the like in the medical industry.
Another object of the invention is to provide a new and improved radial compression mechanism utilizing radially movable dies that produce zero or nearly zero die-to-die gaps.
Another object of the invention is to provide a new and improved radial compression mechanism utilizing radially movable dies that produce a large usable size range.
An advantage of the present radial compression mechanism over prior art is that there is virtually no gap between dies since each of the dies contacts the neighboring dies.
Another advantage of the present radial compression mechanism over the first type of prior art is that there is no tradeoff between die-to-die gaping and the usable range of the central cavity diameter. Therefore, the mechanism can be designed with a relatively large diameter range for a given overall machine envelope.
An advantage of the present radial compression mechanism over the third type of prior art is that the geometry of the dies is much more simple to produce using conventional machining methods and can therefore be made from metallic and non-metallic materials.